The invention relates generally to the field of electromagnetic signal detection and, more particularly, to signal detection using photon-counting detectors.
Photon-counting or particle-counting detectors are used extensively for science, industry and medicine. One example of such a detector is a gas avalanche detector. Recently, a number of new gas avalanche detectors based on parallel grid geometries have been developed. These new designs offer very high counting rate capability as compared to conventional Multiwire Proportional Counters (MWPC). They also offer higher gain, and superior stability and robustness as compared to Microstrip Gas Counters (MSGC). Indeed, this type of detector, when using a 100-micron gap, has demonstrated counting rates on the order of 109 counts/mm2-sec, nearly a million times faster than a conventional MWPC.
One type of parallel grid detector uses an arrangement as shown in FIG. 1, which is a schematic side view of a prior art photon counting detector 10. The detector is configured for use in detecting high energy particles or photons. For example, initial energy component 22 might be an x-ray used in an analysis technique such as x-ray diffraction. A cathode 12 of the detector is a conductive material that is transparent to the energy 22. In this particular detector, a photocathode layer 23 is located on the side of the cathode 12 away from the initial direction of the x-ray. As the x-ray energy passes through the cathode and encounters the photocathode material, it is converted from x-ray energy to a small number of electrons.
Located opposite cathode 12 is an anode 14. The anode is also conductive and is used for collecting electrons that originate at the cathode. One type of anode structure includes two orthogonal serpentine delay lines, as is discussed in more detail below. A voltage differential on the plates 12, 14 is provided by voltage sources 16, 17 and is typically in the range of 0.5-5 kV, the specific amount depending on the desired gain. Often, a conductive mesh 24 is placed between the cathode 12 and the anode 14. Typically, the mesh is a simple cross-hatch of conductive material, although other structures may also be used. The mesh is electrically returned to the voltage source 16, such that a circuit path is defined between the mesh 24 and the cathode 12. Thus, two different voltage differentials are defined by the structure, one across the space 18 between the cathode 12 and the mesh 24, and one across the gap between the mesh 24 and the anode 14. In this example, an electric potential is used in the region 18 that is lower than would be required to cause an avalanche multiplication of the electrons generated at the photocathode layer 23. In contrast, the region between the mesh 24 and the anode has a higher electric potential, which is sufficient to induce avalanche electron multiplication.
In the space 19 located between the anode 14 and the mesh 24 is an active gas material that, in the presence of the electric field generated by the voltage source 17, responds to the introduction of electrons that travel from the photocathode layer 23. With this electric field applied, the electrons from the cathode 12 will induce an avalanche secondary electron multiplication within the gas. An example of an electron multiplication within the detector 10 is given by the graphic depiction of the path 25 of an incident x-ray photon, and the ensuing electron multiplication. As shown, multiple secondary electrons are generated as the initial electron encounters the active gas. These secondary electrons themselves cause the generation of more secondary electrons, and the amplification process continues.
The use of a parallel grid detector allows detection of the electron cloud that results from the avalanche multiplication. For example, as is known in the art, two overlapping serpentine delay lines positioned orthogonal to each other provide a means by which the electron cloud may be located in a two-dimensional detection plane. The overlapping delay lines form a detection grid, the resolution of which is determined by the spacing between the lines, i.e., the xe2x80x9canode pitch.xe2x80x9d As demonstrated in FIG. 2, limits on the anode pitch directly limit the sensitivity of the detector.
FIG. 2 is a schematic view of one serpentine delay line 20 that provides spatial information in one of the two dimensions of the detection grid. It will be understood that the figure is not necessarily to scale, but is intended for instructional purposes only. For each of the parallel portions of the delay line upon which an electron cloud is incident, a signal is generated that is uniquely identifiable relative to that lateral position. Since a second delay line (not shown) has parallel paths that run perpendicular to the parallel paths of the first delay line, signals on these paths provide information relative to the position of the electron cloud in the perpendicular lateral direction of the detection plane. The signals from the two delay lines are detected using a detection circuit 15 (FIG. 1), and are used to determine the region of the detection plane that encounters the electron cloud.
In FIG. 2, regions impacted by two different electron clouds, labeled xe2x80x9cAxe2x80x9d and xe2x80x9cB,xe2x80x9d are represented by circles overlapping the delay line 20. Each of these electron clouds generates detectable signals in the delay line. As shown, electron cloud A overlaps three of the parallel paths of the delay line, thereby generating three different signals at different time delays, and therefore at different determinable spatial positions in a first lateral dimension. However, electron cloud B overlaps only one of the delay line paths. With electron cloud A of FIG. 2, several signals in each of the two dimensions of the detection plane provide sufficient spatial information to calculate a centroid with a resolution more accurate than the anode pitch. However, spatial information provided by electron cloud B is limited by the fact that it overlaps only one delay line path. Thus, it is apparent that the resolution of a detector of this type for relatively small electron clouds will be limited to the anode pitch.
One way to increase the resolution of a delay line detector would be to narrow the pitch between the parallel paths. However, this necessarily increases the length of the delay lines as well which, in turn, significantly increases the signal attenuation. Alternatively, the gap 19 (FIG. 1) between the anode 14 and the mesh 24 can be increased to create a larger drift region within which the electron cloud can expand. However, electron reattachment can occur in this region, the extent of which depends on the gas molecules that are present. Thus, the demand on gas purity in region 19 would be greatly increased, which can be a significant concern for sealed-tube designs that are prone to outgassing over the long term. Moreover, the spacing of the region 19 determines not only the lateral diffusion of an electron cloud, but the longitudinal diffusion as well (i.e., diffusion in the direction perpendicular to the detection plane). More longitudinal diffusion degrades the time resolution of the detector, which can limit the counting rate and, for delay line readouts, degrades the spatial resolution.
In accordance with the present invention, a detection apparatus for detecting an electron cloud in two dimensions includes a resistive layer with a detection plane upon which the electron cloud is incident. The resistive layer is capacitively coupled to a readout apparatus such that interaction of the electron cloud with the resistive layer induces charge in the readout apparatus. The readout apparatus identifies the locations of the charge in a plane that is parallel to the detection plane, and thereby provides an indication of the two dimensional distribution of the electron cloud.
The detection apparatus is preferably part of a parallel grid detector, in which a high-energy photon or particle is amplified using electron avalanche multiplication. In a preferred embodiment, the photon or particle is converted to electrons, which are then accelerated toward an avalanche region. Within the avalanche region, an active secondary electron-emitting material is located and is encountered by the electrons. An acceleration field maintained in the avalanche region is high enough to induce the avalanche of secondary electrons that result in the electron cloud.
In a preferred embodiment, the readout apparatus has a conductive grid, which may consist of two orthogonal serpentine delay lines. Spacing between the resistive layer and the readout apparatus may be selected with regard to the grid. For example, for a given charge, the width of the charge distribution on the readout apparatus is matched to a pitch between conductive segments of the grid. Furthermore, in the preferred embodiment, the resistivity of the layer is used to control the rate of charge dissipation on the anode layer. In particular, the resistivity of the resistive layer is selected relative to the thickness of the anode and the bandwidth of the readout electronics used. The resistivity is selected to be low enough to support the highest bandwidth (i.e., counting rate) of the detector electronics, while still being high enough that the charge can penetrate through the anode layer to the readout plane.